PROJECT SUMMARY Nucleic acid-sensing (NA)-TLRs, notably TLR7 and TLR9, are central players in the pathogenesis of lupus and consequently targeting NA-TLR signaling has been shown to have therapeutic efficacy, but a clinically relevant strategy for specifically inhibiting NA-TLRs has remained elusive. Thus, there is substantial interest in better understanding their basic biology. NA-TLRs are transported from the endoplasmic reticulum to the endolysosomal compartment where they engage ligands and provide signals to activate cells and subsequently the immune system. This project proposes to define the critical endosomal compartments relevant to NA-TLR signaling, B cell activation, autoAb production, and end organ disease. This is based on the premise that endosomal transport of NA-TLRs is similar to the intracellular trafficking system utilized for the biogenesis and function of lysosome-related organelles and that NA-TLRs are a partial form of LROs in NA- TLR-expressing cells. Deficiency of LRO-forming complexes is associated with a rare recessive disorder, called the Hermansky-Pudlak syndrome (HPS) composed of several independent genetic and functional defects that share a common pathway and nearly identical phenotype. This project will use deficiencies in HPS genes as to identify the stages of LRO biogenesis required for NA-TLR signaling and autoimmunity using specifically defects in the AP-3 and BLOC-1 to-3 complexes. This will be accomplished by defining the role of these gene in the development of lupus (aim1), defining the role of AP-3 in TLR signaling in B cells (aim 2), and defining how these four genes function in B cell TLR transport and signaling (aim 3). This project should yield new insights into the biology of NA-TLR signaling within the endolysosomal compartments and the relevance of these compartments to NA-TLR-mediated B cell activation, autoAb production, and systemic autoimmune disease.